Electricity Demand
World electricity demand is expected to double by 2030 and quadruple by 2050. The world electricity demand increase is forecasted to come from developed countries and, to an even larger extent, developing countries. To meet this rapid growth in demand, nuclear power may be a realistic, cost-effective energy source.
Increased energy supply from other sources, such as contribution from natural gas powered generation may be constrained by high and volatile gas prices, greenhouse gas emissions, and concerns over longer-term dependence on unstable sources of supply. Meanwhile, forms of alternative energy (solar, wind, biomass, hydroelectric, etc.) may be useful in satisfying some of the increased demand. They do not, however, scale sufficiently and cannot provide enough additional electric generating capacity in most markets to meet any significant part of the new electricity demand.
Conventional nuclear power plants may also meet part of the added demand. Conventional nuclear power plants, however, have severe obstacles to overcome. These obstacles include: finance capital costs ranging from $3 billion to over $6 billion; uncertainty about waste fuel disposal; and insufficient grid capacity to accommodate large base-loaded power plants.
Coal power plants may also provide some additional supply, but burning mass quantities of coal presents serious political obstacles given the negative environmental impacts.
Needs exist for modular, transportable, self-contained reactors for distributable power to meet the energy demands of the future using new fuels. Any such reactors may be located near consumers, dramatically reducing the need for large and expensive additions to electricity distribution grids. Any long-term, self-contained and clean power sources may have broad applications in markets across the globe.
Traditional Metal Fuels
Metal fuel was the first fuel to be used in nuclear reactors. Later ceramic fuels became common because the early metal fuels were not capable of extended life in a reactor core. Furthermore, there were concerns of excessive fuel cladding interaction at high temperatures.
The deficiencies of the early metal fuels were addressed with a configuration capable of very high reactor exposures where metal fuel alloys prevented excessive fuel cladding interactions (See FIG. 1). FIG. 2 shows a cross-section of an irradiated metal fuel element. The types of metal fuels used in FIG. 1 had significant safety advantages over ceramic fuels because of high thermal conductivities of the metal fuels. Over 100,000 metal fuel elements have been utilized as driver fuel in the Experimental Breeder Reactor (“EBR-II”) and the metal fuel elements have been tested in the Fast Flux Test Facility (FFTF). The fabrication and performance of these metal fuels will be described briefly to allow comparison to the metal fuels of the present invention.
Traditional metal fuels, which have been chosen for several of new domestic and foreign fast reactor concepts, are a cast solid pin of enriched uranium alloy that is sodium bonded inside a low-swelling cladding. The sodium bond fills an appropriately sized gap between the traditional metal fuel and the cladding to facilitate heat transfer at early stages of irradiation. After about 1.5% burnup, the traditional metal fuel itself generally swells to contact the cladding inner diameter and can achieve an excellent heat transfer path. Interconnected porosity can provide a pathway to a gas plenum, which prevents further radial swelling. An extensive performance database exists for traditional metal fuels with over 100,000 metal fuel elements irradiated in EBR-II and tested in both normal and off-normal conditions.
The fuel pin inside the cladding of the older EBR-II and FFTF metal fuel is an alloy of uranium-molybdenum, uranium-zirconium, or uranium-plutonium-zirconium. The fuel pin is injection cast into VYCOR glass molds. Injection casting was chosen for fabrication of the fuel pins because it was useful when used in remote operations for the fabrication of reprocessed fuel. The melting point of the alloys must be less than the softening point of the VYCOR glass molds; thus, the range of possible alloys is limited.
When the fuel pin is broken away from a mold, some of the fuel sticks to the glass mold and must be either treated as a waste product or the glass must be processed to recover the uranium in the fuel. The fuel pin is then loaded into a cladding tube that contains sodium. The cladding tube, with the fuel pin, is heated to melt the sodium. The sodium then fills the gap between the fuel pin and cladding to provide a heat transfer path. The fuel element is vibrated or impacted to remove any voids between the fuel pin and cladding. The fuel element is then inspected with eddy-current or ultrasonic techniques to assure that all the voids have been removed.
End caps are welded on the fuel elements and final inspections are performed. The elements are then placed in a hexagonal steel duct for placement in a reactor. Before the hexagonal steel ducts that contain the elements can be put into a liquid sodium coolant of the reactor, they must be heated from the top downward in a complex fuel loading machine. The reason for the directional heating and melting is that if the ducts were placed directly into the reactor pool, the sodium in the fuel elements would liquefy first from the bottom up and the solid sodium would inhibit adequate thermal expansion of the liquid sodium in the axial direction. The radial expansion of the liquid sodium could deform the cladding.
During the initial stages of irradiation, the generation of fission gas within the fuel pin causes the fuel pin to swell to the inner surface of the cladding. The sodium that was in the gap is displaced into the gas plenum, thus, reducing the volume of the plenum available for released fission gas. The gap between the fuel pin and cladding is designed such that just as the fuel pin reaches the cladding, the pores that form from fission gas in the fuel interconnect. The fission gas is then released into the plenum. Therefore, the stress on the cladding is greatly reduced because the fuel tends to flow back into the open porosity rather than stressing the cladding. This design feature is what allows the metal fuel element to achieve high in-reactor exposures. Once the fission gas is released to the plenum, the driving force for swelling is greatly reduced. The final fuel density for fission gas release prior to cladding contact is 75% or less.
The traditional metal fuels are made by injection casting of sodium bonded metal fuel. These casts greatly limit the range of alloy compositions to be used in the injection cast fuel because of softening of the molds, e.g., VYCOR molds. Additionally, the traditional metal fuel casting process may suffer a loss of volatile components such as americium. Fissile material of the traditional metal fuel tends to cling to the molds, e.g., VYCOR molds. Also, traditional metal fuel processing requires operations to remove bond voids and nondestructive inspections for voids.
Legacy and Future Spent Fuel
An area of concern for nuclear energy is the disposal of light water reactor (“LWR”) spent nuclear fuel (“SNF”). Directly disposing of the LWR SNF requires sequestering for thousands of years. Alternatively, aqueous reprocessing of LWR SNF to remove the long-lived radioactive elements for fission consumption in fast reactors is possible, but expensive. A small fraction of LWR SNF is made up of long-lived actinides (e.g., plutonium, neptunium, and americium) that dominate long-term disposal requirements. In addition, the actinides can represent a potential proliferation risk if entities of concern attempt to recover them for use in nuclear weapons. Therefore, there is a need for a simpler and direct way for handling and disposing of LWR SNF.
Another area of interest for nuclear energy is the recovery of energy still contained in LWR SNF through the presence of the actinides mentioned above. Of the actinides, americium presents a major challenge since it is a major long-term hazard in a repository environment (arising from heat generation and decay to Np237) and its high volatility makes recovery from reprocessing and repackaging into new fuel host difficult. Therefore, there is a need for a means and method for ensuring americium can be recovered for use and energy recovery.